1. Field of the Invention
The present invention relates to a method for, inspecting the surface of a moving strip, in particular rolled sheet metal moving at high speed, as well as to a surface inspection system employing such a method.
2. Discussion of the Background
A first conventional technique for inspecting the surface of sheet metal consists in making a visual examination of the surface of the strip. Such a technique is very demanding for the operator and furthermore makes it necessary to pass the strip through a specific line of a rolling plant, in which it moves at low speed. This technique has the drawback of significantly reducing the productivity of the rolling plant.
Another known surface inspection technique consists in examining the surface condition of the strip automatically, using a photographic camera and a signal processing unit for real-time analysis of the pictures delivered by the photographic camera.
In a method of this type, at least one digital picture of at least one of the faces of the strip is formed, these being made up of a set of successive rows of picture elements which are each assigned a digital value, the said at least one digital picture is filtered in order to detect surface irregularities, by detecting relative variations in the said digital values, and the said at least one filtered digital picture is processed in order to identify the type of surface defect of the strip.
This type of surface inspection technique makes it possible to examine strips moving at speeds very much higher than the speeds used in inspection techniques using visual examination.
However, the digital pictures formed are in the form of successive pictures which are liable to cut certain defects.
There is thus a risk that defects which are cut will be misidentified, and will be identified as being separate defects.
This is also the case when the surface defects are in a segmented form.
One object of the invention is to overcome this drawback.
Another object of the invention is to provide a surface inspection method and a corresponding surface inspection system which make it possible to inspect surfaces moving at high speed.
It therefore relates to a method for inspecting the surface of a moving strip, in order to detect surface defects, comprising the steps consisting in using a photographic means to form at least one digital picture of at least one of the faces of the strip, the digital picture being made up of a set of successive rows of picture elements, each of which is assigned a digital value, filtering the said at least one digital picture in order to detect surface irregularities by detecting relative variations in the said digital values, segmenting the said at least one digital picture into suspect zones in such a way that each suspect zone contains a surface irregularity, and processing the suspect zones in order to identify the type of surface defect corresponding to each irregularity detected, characterized in that the step of segmenting the said at least one picture consists in calculating a first profile of the variation in the digital values of the picture elements in a first direction, in thresholding the said calculated profile so as to define strips of picture elements which each have at least one surface irregularity, in calculating, for each strip defined, a second profile of the variation in the digital values in a second direction, separate from the first direction, and in thresholding the said second calculated profile in order to define the said suspect zones which each contain a surface irregularity.
Thus, since the segmentation of the pictures is no longer carried out arbitrarily, but instead on the basis of the occurrence of surface irregularities, the detects are no longer liable to be cut by the image processing.
The method according to the invention may furthermore have one or more of the following characteristics:
subsequent to the step of segmenting the pictures into suspect zones, for each zone the individual area of portions of the zone which are each made up of picture elements representing an irregularity segment is calculated, and the irregularity segments whose area is less than a predefined threshold are eliminated;
the said picture formation step involves forming a set of pictures of successive parts of the moving strip, in the form of rows and columns of picture elements, and merging the said pictures to form one picture, by storing the said rows in a memory at successive addresses;
the said step of calculating the said first profile involves calculating the sum of the digital values of each column of picture elements, the said step of calculating the said second profile involves calculating, for each strip, the sum of the digital values of each row of picture elements;
subsequent to the step of filtering the stored image, a storage zone is defined in the said memory in response to detection of a picture element with an irregularity, in which storage zone rows of picture elements which are successively delivered by the photographic means and include at least one picture element corresponding to at least one irregularity are stored, the said segmentation step being carried out on each storage zone;
the storage zone opened in the memory in response to detection of a picture element with an irregularity is closed when no picture element with an irregularity has any longer been detected in a predetermined number of picture element rows;
the method furthermore includes a step of detecting storage zones which are of large length in comparison with the number of picture element rows which are stored in each storage zone, with a predetermined threshold for detecting storage zones of large length, the said storage zone being closed in the event that the said threshold is exceeded;
subsequent to the step of segmentation of the image into suspect zones, a group of suspect zones is formed by pairing suspect zones which are contained in successive storage zones, at least one of which is a storage zone of large length, and correspond to the same irregularity, and when the total number of rows of picture elements in the suspect zones of the said group is greater than a default threshold for detecting large length, the image processing step is carried out on one of the said suspect zones in the said group, and the result of the processing is assigned to the other suspect zones in the said group.
The invention also relates to a surface inspection system for implementing a method as defined above, characterized in that it has a system for inspecting the surface of a moving strip for implementing an inspection method, characterized in that it has means for photographing at least one of the faces of the strip, a memory for storing at least one picture of the strip in the form of rows and columns of picture elements which are each associated with a digital value, a circuit for filtering the said at least one digital picture in order to detect surface irregularities by detecting relative variations in the said digital values, and a unit for processing the signals output by the filtering circuit, comprising a first stage for segmenting the said at least one digital picture into suspect zones which each have a surface irregularity, and a second stage for image processing of suspect zones in order to identify the type of surface defect corresponding to the said irregularity, and in that the first stage of segmentation has first means for calculating a first profile of the variation in the digital values of the picture elements in a first direction, and second means for calculating a second profile of the variation in the digital values in a second direction, separate from the first direction.
This system may furthermore have the following characteristics, taken individually or in combination:
the said segmentation stage includes first means for calculating the sum of the digital values of the picture elements in each picture element column in order to construct a first profile of the variation in the digital values, and second means for calculating the sum of the digital values of the picture elements of each row in order to construct a second profile of the variation in the said digital values;
the system furthermore has means for filtering the said profiles, which are associated with the said first and second calculating means, and thresholding means connected to the said filtering means in order to define the said suspect zones;
the system has, arranged at the output of the filtering circuit and connected to the memory, means for managing the addressing of the picture elements in the said memory which are suitable for defining, in the latter, storage zones suitable for recording successive rows of picture elements which each include at least one picture element corresponding to at least one irregularity, the said storage zones each having a maximum capacity;
the processing unit furthermore has means for calculating the number of rows of picture elements of suspect zones which are stored in successive storage zones and correspond to the same irregularity, and means for comparing the said number of rows with a default threshold for detecting large length.